The present invention relates generally to radiocommunication systems and, more particularly, to systems and methods for increasing the capacity and/or coverage area of cellular-type communication systems.
The rapid growth of cellular communication systems has compelled designers to search for ways in which system capacity can be increased without reducing communication quality beyond consumer tolerance thresholds. One way in which increased capacity can be provided is by increasing the efficiency in which the available Cellular spectrum is used, e.g., by changing from analog to digital communication techniques. In North America, this change was implemented by transitioning from the analog AMPS system to a digital system (D-AMPS) which was standardized as IS-54B and later as IS-136. Other technological improvements, such as the implementation of Time Division Multiple Access instead of Frequency Division Multiple Access, have also increased Cellular system capacity.
Even with the implementation of more spectrally efficient technologies, the capacity of cellular communication systems continues to be a concern. Another way in which the capacity of Cellular communications system can be increased is to provide additional spectrum. For example, the FCC originally allocated two blocks of frequencies (i.e., 825-845 MHz (uplink) and 870-890 MHz (downlink)) for Cellular band service in the United States. In 1987, the FCC allocated an additional 5 MHz to each block to increase Cellular systems' capacity. The current Cellular bandwidth allocation in the United States, complete with channel numbering, is illustrated in the tables shown in FIGS. 1A and 1B. Therein, FIG. 1B shows how the transmitter center frequencies can be determined for each channel described in FIG. 1A. Of course, this solution has natural limits since the usable frequency spectrum is finite and other, existing system types already own portions of the usable spectrum.
For example, Land Mobile Radio (LMR) systems are allocated frequency blocks which are contiguous with those of the Cellular band, i.e., 806-824 MHz (uplink) and 851-869 (downlink). The conventional channel assignments for the LMR spectrum are illustrated in the tables of FIGS. 2A and 2B. LMR systems are transmission trunked systems that are commonly used to provide radio communication service between individual units of a particular organization. For example, police departments use a version of LMR (commonly referred to as public service trunked (PST) systems) to communicate between patrol cars and headquarters. Unlike Cellular systems, however, LMR systems have historically been implemented as large independent sites serviced by one (or a few) transmitting base stations, rather than over a wide geographical area serviced by many transmitting base stations as in Cellular systems. At each LMR site, an operator can be allocated a portion of the LMR spectrum within which a fixed frequency pair is selected for use as a control channel while all of the other frequencies can be used for traffic.
In 1994, the FCC announced that the frequency spectrums which have been allocated for LMR, Cellular and Personal Communications Systems (PCS) would be uniformly regulated, such that an operator can use frequencies within the joint bandwidth in any desired manner. Coupled with other regulatory changes, for example those which allow LMR spectrum to be licensed on a wide-area basis rather than a site-by-site basis, it has now become realistic to consider using LMR frequencies in non-conventional ways, e.g., using cellular communication techniques. This usage of the LMR spectrum is termed herein "downbanded cellular (DBC)".
To implement DBC systems which are highly cellular compatible, several challenges must first be addressed. For example, conventional LMR systems operating in the United States have 25 KHz channel widths, whereas Cellular systems operating in accordance with IS-54B have 30 KHz channel widths. Other issues, such as adjacent channel interference, are also a concern.